At the moment structural integrity and high production rate are fundamental characteristics in the aeronautical industry, where competitiveness between airlines is very tough.
Structures with highly integrated boxes are considered to be an important step in the integration of composite structures in the aeronautical field.
Years ago aircraft were mostly or totally built up with metallic components, providing a good performance in terms of mechanical behaviour but, as a drawback, they were penalized in terms of weight.
As the aeronautical industry requires structures that, on the one hand, withstand the loads to which they are subjected, meeting high requirements on strength and stiffness, and on the other hand, are as light as possible, the use of composite materials in primary structures is more and more extended, because with appropriate application of said composite materials it is possible to achieve an important weight saving relative to a design in metallic material.
One of the most important solutions was the use of composite fibre reinforced polymers (CFRP) for major structural parts, achieving important weight savings and cost operation decrease.
In summary, it has been demonstrated that composite materials fulfil the following requirements:                Weight savings.        Cost effective.        Meet structural requisites under aircraft conditions.        Beneficial cost/weight relation.        
A typical horizontal tail plane (HTP) architecture is divided into the following assemblies: leading edge, torsion box, trailing edge, tip, elevator and, in some cases, centre box.
Regarding the torsion box, some designs have been based mainly on a multi-rib structure with two spars (front spar and rear spar) and closed by two stiffened covers. In the case of Airbus A330 and A340, there can be a centre box instead of a centre joining rib as it can be found, for example, in A320. In this case, this structure is assembled and riveted because it has a skeleton with different stiffeners.
Although the first Airbus aircraft such as A300 and A310 were based on a metallic arrangement of ribs and spars, this design was soon replaced by CFRP components produced by different methods such as hand layup, ATL and RTM. The design corresponding to Airbus A350 XWB is based on this last approach, where spars, skins and ribs are monolithic CFRP parts produced by ATL.
The typical CFRP constructions currently used in the HTP torsion box are two-spar constructions comprising a front spar, a rear spar, stringers, skins and several transverse ribs between the front spar and the rear spar, the main function of said ribs being to provide torsional rigidity, to limit the skins and the stringers longitudinally so as to discretize the buckling loads and to maintain the shape of the aerodynamic surface. This structure is stiffened longitudinally by means of the stringers.
Typically, the process for manufacturing a torsion box is considerably manual and is carried out in a number of steps. The different parts (skin, stringers, spars and ribs) forming the box are manufactured separately and afterwards they are integrated by means of co-bonding (specially skins and stringers), and in most cases are mechanically joined with the aid of complicated jigs to achieve the necessary tolerances, which are given by the aerodynamic and structural requirements. This involves different assembly stations and a large amount of joining elements, such as rivets, which entails weight penalties, high production and assembly costs, greater necessary logistic capacity and worse aerodynamic quality in outer surfaces.
Besides that, some of the standard manufacturing processes for boxes imply a separated curing process for the ribs, the spars, the stringers and the skins.
For this reason, there have recently been great efforts to achieve an increasingly higher level of integration in the production of torsion boxes in composite material and thus prevent the aforementioned drawbacks.
For instance, WO 2008/132251 A1 relates to an integrated aircraft structure comprising a multispar torsion box made from composite material without ribs, with several I-shaped or T-shaped longitudinal stringers and spars, that intends to achieve an effective structure as regards strength/rigidity and low weight.
WO 2005/110842 A2, referred to an “Airfoil box and associated method”, discloses an airfoil box that includes two or more half-shell structures that can be integrally formed of composite materials. Each half-shell structure is an integral or unitary member that includes at least a portion of the outer skin of the airfoil as well as stiffener members and connection members. The half-shell structures can be assembled by connecting the connection members with fasteners such as rivets to form the airfoil box.
Some of the prior art proposals try to obtain a high level of integration for the structures and try to avoid the separate manufacturing of the parts forming the box and the use the prepreg technology.
However, some of the current torsion boxes still comprise a high number of components and their manufacturing and assembly processes still involve a high number of operations. Besides that, the prepreg technology used in some of them is costly due to the curing process, which requires an autoclave.